Method and apparatus for preventing ice damage to marine structures

ABSTRACT

A marine well drilling platform or the like having legs extending to the sea floor is protected from ice floes by comminuting devices at the water line for breaking the ice and thereby preventing crushing or overturning of the platform. The comminuting devices employ high velocity impacts against the ice to cause its fracture into chips as distinguished from cutting action. Rapidly rotating or reciprocating mechanisms with large &#39;&#39;&#39;&#39;teeth&#39;&#39;&#39;&#39; for making impact engagement with the ice are employed in separate embodiments. Comminuting devices mounted for sweeping adjacent a mooring buoy in one embodiment open a path through an ice floe for protecting the buoy and a ship moored at the buoy.

United States Patent Schirtzinger 1 June 13, 1972 METHOD AND APPARATUSFOR PREVENTING ICE DAMAGE TO MARINE STRUCTURES Joseph F. Schirtzlnger,Pasadena, Calif.

Air Logistics Col-p Pasadena, Calif.

June 15, 1970 [72] Inventor:

Assignee:

Filed:

Appl. No.:

[56] References Cited UNITED STATES PATENTS 2,545,104 3/1951 Musial ..114/42 X Anna Primary Examiner-Trygve M. Blix Attorney-Christie, Parker &Hale [57} ABSTRACT A marine well drilling platform or the like havinglegs extending to the sea floor is protected from ice floes bycomminuting devices at the water line for breaking the ice and therebypreventing crushing or overturning of the platform; The comminutingdevices employ high velocity impacts against the ice to cause itsfracture into chips as distinguished from cutting action. Rapidlyrotating or reciprocating mechanisms with large teeth" for making impactengagement with the ice are employed in separate embodiments.Comminuting devices mounted for sweeping adjacent a mooring buoy in oneembodiment open a path through an ice floe for protecting the buoy and aship moored at the buoy.

l3 Chins, 11 Drawing Figures PAIENTEDJun 1 3 972 3. 669,052

saw 2 0F 4 METHOD AND APPARATUS FOR PREVENTING ICE DAMAGE TO MARINESTRUCTURES BACKGROUND Within the last few years test wells haveindicated the existence of billions of barrels of petroleum along thenorth coast of Alaska, the delta of the MacKenzie River in Canada, andon the Arctic Islands north of the Canadian Mainland. Much of thisenormous oil reserve is located beneath the sea bed and it becomesdesirable to provide well drilling and oil recovery platforms mounted onthe sea bed for recovering such oil.

Even in the relatively temperate zones surrounding the continentalUnited States, there are substantial problems in maintaining welldrilling platforms in the open ocean. The rigors of an arcticenvironment further complicate the problems of offshore drillingapparatus. One problem not encountered in more temperate zones is thepresence of ice floes that may damage the supports on which the offshoreplatform is mounted. The ice floes are enormous sheets of ice sometimesseveral feet in thickness and extending for many miles. These sheets ofice are driven by wind, and current action so that they move along theocean surface. The movements of ice floes in the Arctic Ocean due towind and current are in the order of nautical miles per day with maximumprobable rates of about 40 nautical miles per day, that is, a littleless than about 2 knots. In some arctic waters, vertical ice movementdue to tides in the order of 30 feet in a 6 hour tidal interval mayoccur. The pressure due to the lateral and vertical movements of the icefloes tend to crush or overturn an offshore structure unless the effectis somehow limited.

In general, drilling platforms are located above the surface of thewater (and ice) and are supported on from one to four columns or legsresting on or driven into the sea bottom or on pads which, in turn, reston the bottom, or in some circumstances, float at a fixed depth belowthe surface of the sea. Piles, driven anchors and weights of variouskinds are used to assist in holding platforms in a fixed position.

In the handling of petroleum, whether recovered from offshore platformsor from wells on land, two principal approaches have been proposed.

One of these involves a pipeline from the north slope of Alaska to anice free Pacific port in southern Alaska. This has substantialdisadvantages that may make installation impractical because of theimpact on the environment. The ecology of northern Alaska depends on ashallow layer which exists in delicate thermal balance between theunderlying permafrost and the atmosphere with its sparse rainfall andwide annual variations of temperature and solar radiation. Anydisturbance of this balance, even as slight as that caused by atrack-type vehicle moving along a slope in summer, has been found insome circumstances to damage the surface vegetation enough to create anerosion gully in which growth is not reestablished. The installation ofa 4-foot pipeline carrying heated oil (no matter how well insulated)could cause significant impact on the environment and, if so, thepipeline construction can be completed only at the expense of theenvironment. Another difficulty with a pipeline is that the entire flowis through a single conduit and damage to the conduit can completely cutoff the flow.

An alternative solution is to carry the oil from the north slope tomarket in large tankers capable of travel through the ice. Such tankersare heavily armored and rely on propulsion and weight to crush their waythrough heavy ice. Once frozen into the ice such a tanker may beincapable of breaking loose without assistance until a substantial thawoccurs. The tankers are advantageous since many would be employed andthe incapacitation of one or more tankers does not completely cut offthe flow of oil.

In order to load tankers, anchored buoyant moorings and platforms may beheld in position in the water by chains and cables to anchors. Duringloading the tanker is moored to or near the mooring buoy and oil istransferred through a conduit from the buoy to the tanker. ln arcticwaters, it is necessary to protect not only the buoy but also the tankerfrom ice floes during the time that it is being loaded.

It is therefore desirable to provide means for protecting either fixedor free flowing structures from ice flees.

BRIEF SUMMARY OF THE INVENTION Thus, in practice of this inventionaccording to a presently preferred embodiment there is provided on astructure positioned in water subject to moving ice floes, meansadjacent the water line for comminuting an ice floe in the regionbetween the advancing ice tide and the balance of the structure whereinthe means for comminuting comprises means for applying interrnittenthigh velocity impacts on the ice.

in a preferred embodiment, the means for comminuting comprises rapidlyrotatable or reciprocatable mechanisms having impacting teeth forengaging the ice and causing shattering thereof. The structure may be awell drilling platform, a pumping platform, mooring buoy or anchoredvessel.

DRAWINGS These and other features and advantages of the invention willbe appreciated as the same becomes better understood by reference to thefollowing detailed description of a presently preferred embodiment whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 illustrates a typical well drilling platform mounted on the seafloor and provided with means for preventing damage by ice floes;

FIG. 2 is a side view of an ice comminuting mechanism on a leg of theplatform of FIG. 1;

FIG. 3 is an end view of the comminuting mechanism of FIG. 2;

FIG. 4 is a perspective view of an alternative ice comminuting mechanismemploying reciprocating motion;

FIG. 5 illustrates in perspective another rotatable ice comminutingmechanism;

FIG. 6 is a side view of an ice comminuting mechanism in the generalstyle of a "chain saw;

FIG. 7 illustrates a detail of teeth of the mechanism of FIG.

FIG. 8 illustrates a reciprocating ice comminuting mechanism;

FIG. 9 illustrates another typical drilling platform provided with acomminuting mechanism mounted on a track;

FIG. 10 illustrates in perspective an ice comminuting arrangement forprotecting a buoy and tanker; and

FIG. 11 illustrates in plan view an ice comminuting arrangement forprotecting an anchored ship.

Throughout the drawings like numerals refer to like parts.

DESCRIPTION FIG. 1 illustrates in perspective a typical well drillingplatform 15 incorporating principles of this invention. In theillustrated embodiment, the platform incorporates various habitablestructures 16 in which the usual operations of drilling, completing, andproducing oil wells are conducted. Typically a high tower I7 is providedon the platform for handling the pipe, drilling tools, and otherequipment employed in the drilling of sub-aqueous oil wells. Theplatform in this embodiment is mounted on three cylindrical legs 18extending downwardly from the platform to a large A-shaped base 19 towhich the legs are securely attached. The base 19 rests on the oceanfloor, and the large weight of the base maintains the entire structurein a fixed position irrespective of forces on the structure due to wind,waves, current, tides, and the like. If desired, the base 19 may be morefirmly secured to the sea floor by means of piles driven into theformations forming the sea floor.

Typically, the platform 15 is mounted a substantial distance above thesurface of the water at the highest tide level so as to be relativelysecure from wave action so that minor storms do not impair the abilityto operate. ln arctic waters, a large ice floe 21 may be encounteredextending for great distances and moving laterally against the legs 18.The direction from which the ice flow 21 presses against the structureis to some degree predictable since there are prevailing winds andcurrents commonly found in most areas. Temporary conditions such asstorms or substantial wind changes may cause the ice flee to travel insomething other than the usual prevailing direction and, therefore, itis desirable to provide ice floe protection equally operable in anydirection.

As the ice floe 21 contacts the legs 18, a substantial lateral pressuredue to the rigid ice pressing against the legs will occur. if the base19 is securely attached to the sea floor the pressure of the ice maycrush or buckle the legs causing the entire structure to collapse. Thelateral pressure of the ice floe against the legs, combined with araising force due to rising tide, will tend to topple the entirestructure. In order to prevent the ice floe from imposing suchundesirable pressures on the legs of the structure an ice comminutingmechanism 22 is provided on each of the legs 18 of the structure.

In the illustrated embodiment the legs of the structure are cylindricaland a comminuting device is readily provided around the cylindrical leg.ln some circumstances the legs of a drilling platform are fabricated ofgirders and other truss-like arrangements of structural steel forproviding rigidity. If desired, the portion of the leg passing throughthe ice zone adjacent the water surface caii be provided with acylindrical sheath on which a comminuting mechanism can be provided, orother arrangements for causing a comminuting device to circumscribe aleg of a drilling platform can readily be provided as will be apparentto one skilled in the art. in some circumstances, a so-called monopod isemployed for a drilling platform, which amounts to one large diametercentral leg on which the platform rests. With such an arrangement, asingle ice comminuting arrangement circumscribing the single leg can beprovided.

FIGS. 2 and 3 illustrate a typical ice comminuting mechanism 22constructed according to principles of this invention. It turns out thatice is an unusual material in that it is one of the very few solidmaterials that break in a taffy-like manner. Materials of this sortbreak in a manner that is dependent more on the rate and concentrationof application of the breaking force than on the magnitude of the forceitself. A steady application of force on ice may cause it to creep orflow in response to the force without breaking. A lesser force appliedat a high rate in a limited region may cause substantial fracturing ofthe ice. Therefore, in order to achieve the maximum ice breaking efl'ectwith a minimum amount of energy expended, it is desirable to produce arapid impact on the ice preferably in a relatively small region. Themaximum shattering effect on the ice is obtained by a plurality ofcutter teeth driven at very high surface speed for intermittent impacton the ice.

In the embodiment illustrated in FIGS. 2 and 3 a plurality of relativelylarge cutter teeth 23 are provided in a generally helical pattern abouteach of a pair of cylindrical mandrels 24. The cylindrical mandrels 24are rotatable at high speed by conventional electric motors 26 or thelike. The teeth 23 are preferably relatively sharp with clearance andback rake for maximum localizing of impact on the ice.

The entire comminuter 22 is mounted on a leg 18 of the drilling platformby way of a cylindrical sleeve 27 that fits around the leg and providestorsional rigidity. The portion of the leg 18 adjacent the water levelis provided with a longitudinally extending key 28 which engages in akey way in the sleeve 27 to prevent its rotation about the leg. Threelongitudinally extending bars 29 are connected to a flange 31 at theupper end of the sleeve 27. The bars 29 extend up to the platform or tosome intermediate structure arranged below the platform and above thehigh tide level. The bars are connected to an elevating mechanism (notillustrated) which can be hydraulically, pneumatically, electrically, ormechanically driven so that the entire comminuting mechanism 22 can beraised or lowered as desired. Thus, for example, it may be desirable toraise the entire mechanism above the water level in order to provideaccess for replacing cutter teeth or performing other routinemaintenance. The bars 29 may also be employed for raising and loweringthe entire comminuting mechanism so that it follows the rise and fall ofthe tide. This permits use of a comminuting mechanism having a mandrel24 only as long as the maximum expected ice thickness rather than onethat includes the maximum ice thickness plus the maximum tidaldifference.

Surrounding the sleeve 27 slidably movable along the length of the leg18, is a second sleeve 32 fitted between the flange 31 at the top of theinner sleeve and a second flange 33 at the lower end of the innersleeve. The outer sleeve 32 is mounted for rotation about the axis ofthe leg relative to the inner sleeve 27 and is driven in rotation by anelectric motor 34 mounted on the end flange 31. The outer sleeve 32 hasflanges 36 at its ends and the two mandrels 24 are mounted between thetwo flanges. Beneath the bottom flange 36 is a rotatable extension ofthe mandrel 24 having a cutter 37 similar to an end mill. If desired thecutter 37 may comprise a plate rotatable at high speed with the mandreland having one or two large impact teeth.

During operation, the entire cutter mechanism 22 is placed in a positionon the length of the leg 18 at approximately the water level so that theupper and lower extent of any ice floe is between the ends of the twomandrels 24. The mandrels are rotated at high speed by motors 26 and atthe same time the outer sleeve 32 is rotated about the inner sleeve 27by the motor 34. The slow rotation of the outer sleeve sweeps therapidly rotating mandrels across the face of an advancing ice floe sothat the teeth 23 intermittently and repeatedly impact on the surface ofthe ice causing fracturing thereof. The sweeping of the impact mandrelsaround the periphery of the leg at the level of the ice floe preventsaccumulation of ice on any side of the leg and continually breaks awaythe ice of the advancing floe for minimizing lateral pressure on thetower leg. The cutters 37 on lower end of the mandrels provide a meansfor lowering the entire comminuting mechanism into the ice in case it isnecessary to raise the mechanism above the top of the ice at some timeduring its operation, say, for example, for routine maintenance. Thesweep of the cutters 37 around the leg permits the entire comminutingmechanism to be lowered through the ice.

The action of the teeth 23 against the ice is in the manner of aplurality of rapid impacts because of the high surface speed of theblades. The repeated high rateimpacts of teeth on the ice causesfracturing in a brittle manner. Such impacting is to be distinguishedfrom the shaving" that occurs as a saw cuts through ice wherein eachsuccessive tooth effectively shaves away a fine swarf from the ice. Inthe illustrated ice comminuter only a few relatively large teeth withclearance and back rake are employed that intermittently strike the iceat high rates of impact in a localized area, thereby causing asubstantial shattering of the ice and removal of chips larger than thepath swept out by the teeth themselves.

Preferably, the teeth 23 on the mandrel 24 are arranged in a helicalpattern and rotated in a direction that in general tends to convey thechips produced by the several teeth in an upward direction so that theyare scattered over the top of the adjacent ice floe or dropped intorelatively open water on the downstream" side of the leg.

In the illustrated arrangement two relatively closely spaced rotatablemandrels 24 are provided; however, it will be apparent to one skilled inthe art that if desired additional mandrels can be employed for morerapid cutting or to provide increased redundancy in the cutting system.Such additional cutters can be mounted adjacent the illustrated cuttersor may, for example, be placed on the opposite side of the flanges 36from the illustrated cutters. Many other such arrangements will beapparent to one skilled in the art.

HO. 4 illustrates another embodiment of ice comminuting mechanism usefulin practice of this invention. As illustrated in this embodiment a leg18 of the tower is provided with a circumscribing sleeve 41. A flange 42is provided at each end of the sleeve 41 (only the upper flange isillustrated and it will be understood that the lower flange issubstantially identical). Rods 43 connected to the flange 42 extendupwardly for raising and lowering the comminuting mechanism to followthe ebb and flow of the tide. A helical groove 44 on the inside sur'face of the sleeve 41 engages a pin 45 on the outside of the cylindricalleg 18. Raising and lowering of the comminuting mechanism thereforecauses the sleeve 41 to be rotated about the cylindrical leg.

Mounted on the flange 42 are a plurality of hydraulic or pneumaticactuators 46 to which are connected longitudinally extending mandrels47. In the illustrated arrangement only two such actuators and mandrelsare illustrated; however, it will be understood that additional suchcombinations can be provided over much or all of the periphery of thesleeve 41 as may be desired. The mandrels 47 are provided with aplurality of relatively large teeth 48 extending outwardly from themandrels. In operation the actuators 46 intermittently stroke in alongitudinal direction so that the mandrels are reciprocated up and downat a high rate. The teeth 48 impact against ice in a floe causingshattering thereof in substantially the same manner as the high speedteeth 23 on the rotating mandrel of the embodiment of FIGS. 2 and 3.Raising and lowering of the sleeve 41 by means of the rods 43 causes itto be rotated about the leg due to engagement of the groove 44 with thepin 45. This causes the reciprocating mandrels to sweep out an arcuatepath and maintain the entire periphery of the leg free of ice pressure.

FIG. 5 illustrates in perspective another embodiment of rotating icecomminuter incorporating principles of this invention. As illustrated inthis embodiment, a plate 51 circumscribes a leg 18 of a drilling tower.The plate 51 is prevented from rotating relative to the tower leg by akey 28 and can be raised and lowered along the length of the tower legas desired by means of longitudinally extending rods 52. Mounted beneaththe plate 51 is a rotatable ring 53 on the periphery of which aresecured a few large impacting teeth or bars 54. A tangentially extendingnozzle 55 (or if preferred a plurality of such nozzles) is provided onthe periphery of the ring 53 so that high velocity fluid such" ascompressed air, exhaust gases, steam, or water can be ejected forcausing the entire ring 53 to rotate at high speed.

Mounted beneath the upper rotatable ring 53 is a substantially similarrotatable ring 56 having peripheral teeth or bars 57. A tangentiallyextending nozzle 58 through which high velocity fluid can be ejected isprovided on the periphery of the ring 56 for causing rotation in adirection opposite to the direction of rotation of the upper ring 53. Itis preferred to skew the teeth 54 and 57 in a direction that tends tothrow the chips produced in a generally upward direction for clearingthe ice chips away from the kerf cut by the teeth on the rapidlyrotating rings.

It will be apparent to one skilled in the art that other tootharrangements and driving mechanisms can be employed in such anembodiment or, if desired, a different number of rotatable rings can beemployed.

FIG. 6 illustrates in side view a portion of another ice comminutingmechanism useful in practice of this invention. As illustrated in thisembodiment, a roller chain 61 extends in a generally vertical directionalong a chain backing member 62, illustrated only schematically in FIG.6 (such backing members are well known in the so called chain saws). Theroller chain 61 passes over a drive pulley 63 mounted on a shaft 64driven by some conventional motor (not shown).

As seen in the side view of FIG. 6, and in greater detail in the faceview of FIG. 7, two types of teeth are mounted on the roller chain. Afirst tooth 65 is relatively long and narrow, that is, it extends asubstantial distance from the chain in the plane of the chain, and doesnot extend a substantial distance out of the plane of the chain. Theseteeth are preferably cocked forwardly for providing back rake with easeof sharpening. A

second type of tooth 66 alternating with the first type of tooth 65 isrelatively broad in a direction normal to the plane of the chain, and isrelatively short in a direction in the plane of the chain. These teethalso have both clearance and back rake for localized impacting .on ice.Thus, as the chain is rapidly rotated by the drive pulley 63, the iceencountered by the teeth is alternately impacted by the long narrowteeth 65 and the short wide teeth 66. Thus, the portion of ice impactedvaries from time to time, tending to provide greater shattering effect,and further the relatively wide teeth 66 tend to remove the chips fromthe kerf as they are produced with greater effectiveness than therelatively long teeth 66, which provide greater shattering effect. Achain driven type of ice comminuter can be employed in substantially thesame arrangement that the rapidly rotating or reciprocating icecomminuters are employed.

FIG. 8 illustrates in perspective an additional embodiment of icecomminuter constructed according to principles of this invention. Asillustrated in this embodiment, the comminuter is mounted on a leg 18 ofa drilling platform or the like. A rack 68 is provided along at least aportion of the leg 18 so that a pinion (not shown) can be used to drivea circumscribing ring 69 along the length of the leg. During operation,the ring 69 is slowly raised and lowered along the length of the leg inthe zone that may encounter ice. Mounted on the ring 69 is a surroundingring 70 which is slowly rotated about the inner ring 69.

Mounted on the periphery of the outer movable ring 70 are a plurality oflarge ice comminuting teeth 71, each extending outwardly from the outerring. In operation the teeth 71 are rapidly reciprocated in a verticaldirection, that is, along the length of the leg 18 so as to rapidlystrike the ice with a high rate of impact. The teeth are reciprocated byconventional pneumatic devices substantially the same as those employedin jack hammers". The shattering effect of the teeth 71 on the ice asthe ring is raised and lowered and slowly rotated maintains the areasurrounding the leg 18 free from ice floe pressure.

in addition to the radially extending teeth 71 there may be provided aplurality of longitudinally extending teeth 72 at the upper and lowerends of the ring 70 so that if ice forms above or below the ring whileit is inactive or withdrawn, the ice can be shattered for inserting thering through the ice and permitting the circumferential teeth 71 toproperly perform. The longitudinally extending teeth 72 are selectivelyreciprocated when desired in order to first cause the ring to passthrough a layer of ice. In the illustrated embodiment, thelongitudinally extending teeth 72 are illustrated as relatively bluntcones in order to introduce a localized impact on the ice; however, itwill be apparent to one skilled in the an that sharper cones, bluntends, or knurled ends can be provided on the longitudinally extendingteeth as desired. It will be apparent also that in lieu of separatelyreciprocatable teeth an entire ring 70 can be longitudinallyreciprocated in order to effect ice comminution. It is preferred,however, to employ a plurality of independently reciprocatable teethsince higher impact rates can be obtained and asynchronous reciprocationof the individual teeth minimizes stresses on the central ring 69.

The preferred ice comminuters have been illustrated in relation to asingle leg of an oil drilling platform. It should be apparent, however,that the high impact rate comminuters can also be employed in otherarrangements. Thus, for example, HO. 9 illustrates in perspective an oildrilling platform 75 or the like on which a drilling tower 76 ismounted. The platform 75 is supported on a plurality of legs 77extending from above the water surface through an ice floe zone to asupporting structure (not shown) in engagement with the sea floor. Ifdesired, the legs 77 may be connected to a subfloating structure ratherthan to the sea floor.

Mounted on the legs '77 and extending around the entire periphery of thetower is a horizontal track 78 on which is mounted a movable carriage79. The carriage 79 can be any of a broad variety of mechanisms fortraveling around the track 78 at a controlled rate. Mounted on thecarriage 79 are a plurality of ice comminuters 80 extending from thecarriage down through the ice floe zone. The comminuters 80 can be anyof the types illustrated in the hereinabove described embodiments ofFIGS. 2 through 8.

In operation, the carriage 79 is caused to travel back and forth alongthe track 78 on the side or sides of the platform from which an ice floeis approaching. The comminuted ice produced passes beneath the platformand continues downstream with the balance of the ice floe. If desired,the carriage 79 can be traversed completelyaround the platform in orderto assure that the ice does not freeze together again before leaving theregion of the platform and for minimizing lateral pressure in directionstransverse to the direction of movement of the ice floe.

FIG. illustrates in perspective an arrangement for an ice comminuterparticularly useful for protecting a mooring buoy either alone orwhen atanker is moored thereto, at which time the comminuter also protects thestationary tanker from the ice floe. In accordance with this embodiment,there is provided a semi-submerged mooring buoy 83, such as, forexample, similar to the buoy described and illustrated in US. Pat. No.3,466,680. The principal portion of such a buoy 83 floats beneath thewater surface with the buoyancy adjusted so that a mast 84 extends abovethe water surface with navigational aids 85 mounted at the top.

A peripheral track 86 around the buoy has a carriage 87 supporting apipe 88 or similar conduit to which oil is pumped to a tanker 89. Thepipe 88 connects to a flexible hose 91 so that the motion of the tanker89 relative to the buoy can be accommodated. Preferably, a mooringhawser 92 is provided between the mooring buoy 83 and the tanker 89 formaintaining the tanker within a selected distance from the buoy. Thepipe 88 connects to a fluid swivel 93 at the base of the mast 84 so thatas the ship weathervanes around the buoy due to the changing influencesof current and wind, the pipe can follow the ship motion without damageto the oil conducting conduit.

In the illustrated arrangement a second track 94 is provided below thetrack 86 and a second carriage 96 is mounted on the two tracks so thatit, too, can travel around the entire periphery of the mooring buoy, asdriven by a motor (not shown). Mounted on the carriage 96 is ahorizontal underwater beam 97 extending to a float 98 so that as thecarriage 96 traverses around the periphery of the buoy the float 98 iscaused to swing in a circular arc around the center of the buoy. Thefloat 98 is illustrated extending above water level, however, it will beapparent that it, like the buoy may be largely submerged if desired.

Mounted on the float 98 are a plurality of ice comminuters 99 providinghigh rate impacts on the edge of an ice Hoe 101. The ice comminuters 99are preferably any of the embodiments hereinabove described andillustrated in FIGS. 2 through 8.

In operation, the tanker 89 normally floats in a position relative tothe buoy 83 downstream" from the direction of travel of the ice floe 101since the same general influences are acting on the ice floe and on theship. There may, of course, be situations where a temporary lateral windmay afiect the ship at a different time than the ice floe, and it may benecessary to temporarily disengage the tanker from the mooring buoyunder such conditions. Under normal conditions, however, the tanker isdownstream" from the buoy.

In such a situation the carriage 96 connected to the float 98 is drivenback and forth along an arc I02 approximately centered on the directionof ice floe so that the ice advancing towards the buoy and ship isshattered by the comminuters 99. The shattered ice continues to movewith the general movement of the ice floe and passes along a pathstraddling the buoy and tanker. In the illustration of FIG. 10, the icefloe 101 is illustrated as a solid sheet, as it most often is, and theregion occupied by shattered ice fragments is illustrated as open water.It will be recognized, of course, that the shattered ice issubstantially fluid as compared with the solid ice floe but that openreaches of water will seldom be encountered, and the open waterillustrated is only for purposes of clarity in the drawings. Theshattered ice remains as discrete fragments for a considerable distance"downstream" from the comminuters and the ship is, in effect, in openwater. If desired, an additional ice comminuter can be provided fortravel on the carriage 96 or on the tracks 86 and 94 separate from thecarriage so that during extremely adverse conditions the buoy may bemaintained free of ice at all times. This is merely an added precautionfor extremely adverse conditions since the buoy must remain in place atall times, whereas a tanker may leave and rely on its propulsive powerand weight to counter an ice floe.

FIG. 11 illustrates in plan view an ice comminuting arrangement suitablefor protecting an anchored ship or the like from the effects of an iceflow. As illustrated in this embodiment, a conventional ship 103 isanchored by one or more anchored chains 104 dropped tothe sea floor. Theship 103 is provided with an ice comminuting mechanism that provides achannel of open water 105 in an otherwise solid sheet of an ice floe106. The channel of open water 105 is like the open channel provided bythe mechanism illustrated in FIG. 10, and it will be understoodconstitutes a region of unconsolidated ice chips in an otherwise solidice floe rather than a stretch of open water.

The ice comminuting mechanism comprises a forwardly extending truss-likebeam 107 attached to the bow of the ship I03. Connected to the forwardlyextending beam I07 is a transverse beam 108, the length of which isapproximately the same as the maximum width of the ship. The transversebeam 108 in the illustrated embodiment is a straight member far enoughforward of the bow that it cannot in any way interfere with the anchorchains 104. The transverse beam 108 can, if desired, be provided withfloating members for supporting at least a part of the weight of thebeams, or if desired the beams can be connected to the ship withsufficient structural rigidity to support their own weight. It will asobe apparent that if desired the transverse beam 108 can be a curvedmember.

Mounted on the transverse beam I08 is a carriage I09 capabio of movingalong the length of. the beam in a controlled manner. Mounted on thecarriage 109 are three ice comminuters 110, such as, for example, thetype illustrated in FIGS. 2 and 3. These ice comminuters 110 rotate athigh speed, thereby causing their teeth to impact on the face of theadvancing ice floe 106 for shattering it and creating the open stretchof water 105 in which the ship rides. The end ice corn minuters 110 areprovided somewhat outwardly from the carriage I09 and the carriage iscapable of travel to a position where a portion of it extends beyond theend of the beam I08 so that the width of the open stretch of water isgreater than the length of the transverse beam 108.

With an ice comminuter as provided in either of FIGS. 10 or 11, themoving mechanism is mounted on a floating structure and no specialmechanisms are required for raising and lowering the comminuter inresponse to the ebb and flow of tide. It will also be recognized that ifdesired the comminuter provided in the embodiment of FIG. 11 can bemounted directly on a track running around the bow of the ship so longas the track is provided at a sufficient distance from the how that theanchor chains are not interfered with.

Although the invention has been described and illustrated in relation tostructures particularly suitable for oil drilling and recovery, it willbe apparent to one skilled in the art that principles of this inventionare applicable to other structures subject to damage due to ice floes.Many other modifications and variations will be apparent to one skilledin the art, and it is therefore to be understood that within the scopeof the appended claims the invention may be practiced otherwise than asspeciftcally described.

What is claimed is:

1. Apparatus for preventing ice damage comprising:

a structure fixedly positioned in water subject to moving ice meansconnected to the structure adjacent the water line for comminuting icefloe in a region between the advancing ice floe and the balance of thestructure, said means for comminuting comprising means for applying highvelocity impacts on local areas ofthe ice; and

means for traversing the means for comminuting about at least a portionof the structure adjacent the water line.

2. An apparatus as defined in claim 1 further comprising:

means for raising and lowering the means for comminuting relative to thestructure for engaging the means for comminuting with the entirevertical extent of an ice floe.

3. An apparatus as defined in claim 1 wherein the means for traversingcomprises a track extending around the periphery of the structure; and

a carriage mounted on the track for supporting the means forcomminuting.

4. An apparatus as defined in claim 1 wherein the structure comprises aplurality of vertically extending legs passing through the water surfaceand the means for traversing comprises means mounted on each of the legsfor traversing a means for comminuting about that leg.

5. An apparatus as defined in claim 1 wherein the means for comminutingcomprises:

a rotatable mandrel;

means for rotating the mandrel at high velocity; and

a lurality of teeth on the mandrel for impacting against an 6. Anapparatus as defined in claim 1 wherein the means for comminutingcomprises:

a vertically reciprocatable mandrel;

means for reciprocating the mandrel at high velocity; and

a plurality of teeth on the mandrel for impacting on ice.

7. An apparatus as defined in claim 1 wherein the means for comminutingcomprises:

a vertically movable member; and

a plurality of reciprocatable teeth projecting from the movable memberfor applying high velocity impacts to ice.

8. A combination comprising:

a mooring buoy positioned in water subject to moving ice floes;

a track on the periphery of the mooring buoy;

a carriage on the track;

a substantially horizontal beam extending from the carriage;

a buoyant member connected to the beam; and

means mounted on the buoyant member for oscillation along an arc aboutthe center of the buoy adjacent the water line for comminuting ice floein a region between an advancing ice floe and the balance of thestructure, said means for comminuting comprising means for applying highvelocity impacts on local areas of the ice.

9. A well drilling platform comprising:

a work platform positioned above a water surface subject to moving icefloes;

means on the platform for drilling and completing oil wells and thelike;

a plurality of vertically extending legs from the work platform to thesea bed;

ice comminuter means adjacent the water surface and circumscribing allof the legs for comminuting ice, said means for comminuting icecomprising means for applying high velocity impacts on the ice.

10. A platform as defined in claim 9 wherein the means for comminutingcomprises:

an ice comminuter mounted on each of the legs; and

means for traversing the ice comminuter on each leg around the leg.

11. A platform as defined in claim 9 further comprising:

a track extending around the platform and outside of all of the legs;

a carriage mounted on the track for traversing around the entireplatform; and wherein the means for comminuting is mpuntedon thecarriage 12. A mooring buoy for positioning in waters su ect to movingice floes comprising:

a floating buoy;

a track around the buoy;

a carriage mounted on the track for motion around the buoy periphery;and

means connected to the carriage for comminuting an advancing ice floe.

[3. A mooring buoy as defined in claim 12 wherein the means forconnecting the means for comminuting to the carriage comprises:

a beam extending laterally from the carriage; and

a buoyant structure on the beam and supporting the means forcomminuting, whereby as the carriage moves about the track the means forcomminuting ice oscillates along an arc about the center of the buoy forcomminuting ice in a path substantially larger than the width of thebuoys l i k

1. Apparatus for preventing ice damage comprising: a structure fixedlypositioned in water subject to moving ice floes; means connected to thestructure adjacent the water line for comminuting ice floe in a regionbetween the advancing ice floe and the balance of the structure, saidmeans for comminuting comprising means for applying high velocityimpacts on local areas of the ice; and means for traversing the meansfor comminuting about at least a portion of the structure adjacent thewater line.
 2. An apparatus as defined in claim 1 further comprising:means for raising and lowering the means for comminuting relative to thestructure for engaging the means for comminuting with the entirevertical extent of an ice floe.
 3. An apparatus as defined in claim 1wherein the means for traversing comprises a track extending around theperiphery of the structure; and a carriage mounted on the track forsupporting the means for comminuting.
 4. An apparatus as defined inclaim 1 wherein the structure comprises a plurality of verticallyextending legs passing through the water surface and the means fortraversing comprises means mounted on each of the legs for traversing ameans for comminuting about that leg.
 5. An apparatus as defined inclaim 1 wherein the means for comminuting compriseS: a rotatablemandrel; means for rotating the mandrel at high velocity; and aplurality of teeth on the mandrel for impacting against an ice floe. 6.An apparatus as defined in claim 1 wherein the means for comminutingcomprises: a vertically reciprocatable mandrel; means for reciprocatingthe mandrel at high velocity; and a plurality of teeth on the mandrelfor impacting on ice.
 7. An apparatus as defined in claim 1 wherein themeans for comminuting comprises: a vertically movable member; and aplurality of reciprocatable teeth projecting from the movable member forapplying high velocity impacts to ice.
 8. A combination comprising: amooring buoy positioned in water subject to moving ice floes; a track onthe periphery of the mooring buoy; a carriage on the track; asubstantially horizontal beam extending from the carriage; a buoyantmember connected to the beam; and means mounted on the buoyant memberfor oscillation along an arc about the center of the buoy adjacent thewater line for comminuting ice floe in a region between an advancing icefloe and the balance of the structure, said means for comminutingcomprising means for applying high velocity impacts on local areas ofthe ice.
 9. A well drilling platform comprising: a work platformpositioned above a water surface subject to moving ice floes; means onthe platform for drilling and completing oil wells and the like; aplurality of vertically extending legs from the work platform to the seabed; ice comminuter means adjacent the water surface and circumscribingall of the legs for comminuting ice, said means for comminuting icecomprising means for applying high velocity impacts on the ice.
 10. Aplatform as defined in claim 9 wherein the means for comminutingcomprises: an ice comminuter mounted on each of the legs; and means fortraversing the ice comminuter on each leg around the leg.
 11. A platformas defined in claim 9 further comprising: a track extending around theplatform and outside of all of the legs; a carriage mounted on the trackfor traversing around the entire platform; and wherein the means forcomminuting is mounted on the carriage.
 12. A mooring buoy forpositioning in waters subject to moving ice floes comprising: a floatingbuoy; a track around the buoy; a carriage mounted on the track formotion around the buoy periphery; and means connected to the carriagefor comminuting an advancing ice floe.
 13. A mooring buoy as defined inclaim 12 wherein the means for connecting the means for comminuting tothe carriage comprises: a beam extending laterally from the carriage;and a buoyant structure on the beam and supporting the means forcomminuting, whereby as the carriage moves about the track the means forcomminuting ice oscillates along an arc about the center of the buoy forcomminuting ice in a path substantially larger than the width of thebuoy.